I agree that cardio AND weights are important.
YOu need the weight work to build shapely muscle under the fat you want to get rid of. And more muscle on you means a higher metabolism 24 hours a day (though how much higher seems open to debate).
If you ever see people who JUST do cardio, they are often "skinny fat" because too much cardio can eat up muscle tissue, so they end up "thin," but in a scrawny, unhealthy-looking way (though it's true that some forms of cardio have resistance benefits: kickboxing, swimming, etc.).
To specifically "lean out" (if you have the muscle mass you want and are just looking to get rid of fat), I suggest emphazising full-body weight workouts, circuit (weight + cardio) workout, and cardio. And make one of your cardio workouts in the week an interval workout like one of the IMAXes. Interval workouts are a more efficient way of burning more calories.
Of course, nutrition factors in here as well. It's easier to avoid eating 100 calories than it is to burn off 100 calories with exercise.